Stabilizing apparatus for tremolo system for string instrumen

ABSTRACT

A few-moded fiber device has several discrete sections of few-moded fibers that are separated by mode converters, with each mode converter accomplishing mode conversion between one or more pairs of modes. The mode conversions can be accomplished using a sequence, such as a periodic or cyclic sequence that would cause (1) a signal wave launched with any mode to assume every other mode for one or more times; (2) the number of times the signal remains in any modal state is substantially the same; and (3) the net signal gain or loss or group delay of the input signal is substantially the same regardless of the state of input mode. A laser few-mode amplifier is provided. An optical transmission system is also provided.

STATEMENT OF RELATED CASES

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 61/521,902, having the title “Few-moded fiberdevice employing mode conversion,” filed on Aug. 10, 2011, the entiretyof which is incorporated herein by reference.

BACKGROUND

There has been significant interest in using higher-order modes ofoptical fibers for multiplexing data to enhance the bandwidth oftransmission. Since various modes display various shapes in the plane offiber cross section, this is commonly known as spatial divisionmultiplexing (SDM), or mode division multiplexing. To compensate theloss in the fiber link, fiber amplifiers capable of amplifying all themodes of interest are required. SDM transmission links that are based onhigher order modes depend on multiple-input, multiple-output (MIMO)signal processing, which demands for small modal dispersion of the link.

Rare-earth doped or nonlinear (Raman) fibers can be made with core sizeslarge enough to support various modes such as LP₀₁, LP₁₁, LP₀₂, LP₂₁,etc. Since the radial field distribution of these modes are different,the overlap factor Γ, of the electrical field of individual modes withthe gain region can differ significantly. The overlap factor is definedas:

Γ = ∫_(R 1)^(R 2)∫₀^(2π)E²r r φ/∫₀^(∝)∫₀^(2π)E²r r φ

where, R1 and R2 are the radii of the circular region within whichvarious signal modes experience amplification. In a typical amplifierwith a cylindrical doped region, R1 is typically zero.

As Γ depends on the transverse mode of the signal wave, the gainexperienced in a few mode amplifier (both the rare-earth andRaman/Brillouin amplifiers) by the different modes becomes different.This difference in gain can be problematic when used to amplify variousmodes carried by a few-moded fiber in a space division multiplexed (SDM)transmission system. It has not yet been possible to split and/orcombine various fiber modes without causing significant losses in orderfor them to be amplified by separate C-, L-band amplifiers (schemessimilar to that used in C+L band signal amplification). Another problemassociated with SDM transmission systems involving few-moded fiber linksis that the group index n_(g) and thus group delay of each mode can besignificantly different.

Therefore, there is a need for a few-mode fiber amplifier or amplifyingdevice that will ensure equal gain to all the modes of the fiber.

SUMMARY

Embodiments of the present invention depict a few-moded optical fiberdevice to process an input optical signal containing N modes, where N isan integer greater than or equal to 2, including an input few-modedfiber enabled to receive the input signal, at least N mode convertersarranged in a pre-determined order, wherein a first mode converter ofthe N mode converters is coupled to the input few-moded fiber, whereineach mode converter transforms one modal state of the N modal states toa different mode, a few-moded connecting fiber between each of the Nmode converters, and an output few-moded fiber coupled to a last modeconverter of the N mode converters to provide an output optical signalcontaining the N modal states, wherein each of the N modes in theoptical output signal are characterized by a substantially identicaltransmission parameter relative to corresponding N modes of the inputsignal, such as gain, loss, group delay and/or dispersion.

DESCRIPTION OF DRAWINGS

FIGS. 1 and 2 illustrate intensity distribution for low order modes in astep index fiber;

FIGS. 3-5 illustrate radial electric-field distribution of certain modesof a step index fiber;

FIGS. 6-8 illustrate a fractional optical power outside of a fiberradial position;

FIGS. 9 and 10 illustrate a gain equalized few-moded fiber device;

FIG. 11 illustrates periodic index perturbations;

FIG. 12 illustrates conversion between modes using microbending;

FIG. 13 illustrates periodicity of LPG/microbending related to modeconversion;

FIGS. 14 and 15 illustrate LPG/microbending as a function of wavelength;

FIG. 16 illustrates a mode conversion sequence;

FIGS. 17 and 18 illustrate cyclic mode conversion;

FIG. 19 illustrates the increasing length of section of gain fiber;

FIG. 20 illustrates an example of employing multiple pump sources;

FIG. 21 illustrates a multimode signal and pump combiner;

FIG. 22 illustrates multimode pump combiner with cladding pumping;

FIGS. 23 and 24 illustrate a multi-moded fiber laser; and

FIG. 25 illustrates a multi-channel mode division multiplexed opticaltransmission system

DESCRIPTION OF THE INVENTION

To address these and other problems, a novel architecture is provided ofa few-moded fiber device or link involving a multiple modetransformation process along its length. This ensures equal opticalproperties (e.g., gain, absorption, group delay, or dispersion) for allthe relevant input modes.

The few-moded fiber device provided herein incorporates discrete modeconverters and a few-moded fiber section (passive and active) placedalternately such that modes are converted from one to another as anoptical signal travels from one fiber-section to the next. This is donein such a way that the signal wave launched in any mode is amplified (orabsorbed in the absence of pump) by the same amount or experiences thesame amount of group delay or dispersion. In this application, a modeconverter serves the purpose of changing one waveguide or fiber mode(e.g., LP_(mn) to LP_(xy), and vice versa).

More specifically, the fiber device has several discrete sections offew-moded fibers that are separated by mode converters, with each modeconverter accomplishing mode conversion between one or more pair(s) ofmodes. The mode conversions can be accomplished using a sequence, suchas a periodic or cyclic sequence that would cause the following: (1) asignal wave launched in any mode to assume every other mode one or moretimes; (2) the number of times the signal remains in any modal state isthe same; and (3) the net signal gain or loss or group delay ordispersion of the input signal is the same regardless of the state ofinput mode. The number of modes involved in the modal conversions can beequal to or less than the modes supported by the few-mode fibers, andmust include at least all the modal states of the input signal that areof interest. Practically speaking, there may be significant fabricationimperfections, unintentional perturbations, etc., such that delay, gainand loss cannot be completely independent of input mode. The disclosedschemes provide statistically greater mixing of modes and thusstatistically greater independence of input launch than would beachieved otherwise. Details of the design features of this few-modedfiber device are provided herein.

In accordance with various aspects of the present invention thefollowing devices, fibers, and features of fibers are provided:

a) A few-moded fiber device that incorporates at least two or moredistinct mode converters, arranged in a preferred sequence, andseparated by certain length of few-moded fiber or fibers (passive oractive gain fiber), each fiber being capable of supporting multiplehigher-order modes.

b) In the aforesaid few-moded fiber device, the mode-conversion sequenceis such that i) at each of the mode-converters (at a given location),one or more modes are interchanged between one (or more) pair of modes,ii) after each mode-conversion process, all modes remain different fromone another, iii) at the end of the sequence, the modes are restored tothe original state (i.e., substantially the same as at the input), andiv) the overall gain or loss, or group delay or dispersion associatedwith each launched mode is substantially the same.

c) The length of the gain fibers separating the mode-converters is suchthat the gain (or loss) experienced by any particular mode (e.g., LP₀₁)in each gain section will be substantially the same.

d) The length of the passive fibers separating the mode-convertersshould be such that the group delay experienced by any particular mode(e.g., LP₀₁) in each gain section will be substantially the same.

e) Item (c) further implies that in the absence of gain saturation,(e.g., when the gain per unit length is substantially the same), thelength of gain sections will be nominally equal. In the presence ofsignificant gain saturation occurring along the gain fiber, the span ofsuccessive amplifying sections can be made different (e.g., increasinglylonger) such that any given mode is amplified by substantially the sameamount in each of the gain sections.

f) Item (d) further implies that when the few mode fibers located inbetween the converters have substantially the same group delay per unitlength, the length of the sections will be nominally equal.

g) The mode-conversion between different modes (e.g., LP₀₁ , LP₁₁, andLP₀₂) is performed using long period gratings (LPG) or by applyingperiodic micro bends or bulk spatial phase modulators, or volume phasegrating or photonic lattice (e.g., 1D). For conversion between radiallysymmetric modes (e.g., LP₀₁⇄LP₀₂), an LPG is preferred. For conversion,involving radially asymmetric modes (e.g., LP₀₁⇄LP₁₁ or LP₀₂⇄LP₁₁),periodic index variation employing periodic microbends are preferredover other conversion means.

h) In one embodiment of the present invention, each gain section ispumped using a common pump wave launched in the forward or in thebackward direction or in both direction. Alternately, each gain sectionis pumped by individual pump waves to suppress pump depletion.

j) The gain section in one embodiment of the present invention is onethat is doped with one or more rare earth elements (for rare earthamplifiers), or doped with other elements such as germanium to enhancenonlinearity (for Raman or Brillouin amplifiers).

k) In one embodiment of the present invention, the fiber sectionsconnecting the mode converters are passive fibers instead of gainfibers. This will ensure that the net modal dispersion becomes zero.

l) In one embodiment of the present invention, the gain fibers areseparated by isolators to suppress back-reflections and suppressamplified spontaneous emission (ASE) noise.

m) In one embodiment of the present invention, the few-mode amplifier isterminated by Bragg gratings or reflectors to operate as lasers thatwill simultaneously oscillate with different spatial modes.

Few-moded or few-mode (also known as higher-order mode fiber or HOM)fibers are known to support fiber modes with distinct radial/transversepower distribution as illustrated in FIGS. 1 and 2 which provide each indiagram a schematic of intensity distribution for some low order modesin a step index fiber, with a circular core 101 in FIG. 1 and anelliptical core 201 in FIG. 2.

While LP₀₁ and LP₀₂ are radially symmetric and peaked at the center,LP₁₁ has multiple lobes. Like the fundamental mode LP₀₁, higher-ordermodes can exist with two orthogonal polarizations.

Further, radially asymmetric modes such as LP₁₁, can have differentorientation of the lobe pattern. In a fiber with circular cores, twodifferent orientations of such lobes can exist. By using an ellipticalcore as shown in FIG. 2, however, it is possible to make LP₁₁ modesnon-degenerate, so that accidental coupling between these modes isdrastically reduced.

In FIGS. 3, 4, and 5, the e-field distributions for the LP₀₁, LP₀₂,LP₁₁, and LP₁₂ modes for a common step index fiber are illustrated atthree different wavelengths, i.e., 1550 nm (FIG. 3), 1480 nm (FIG. 4),and 980 nm (FIG. 5). The step index fiber has an index difference of0.0175 (measured at 632 nm) and core diameter of 10 micron. For the LP₁₁and LP₂₁ modes, which have of multiple lobes, the graphs show the fieldin the radial direction along which the field intensity is the maximumat different wavelengths.

In FIGS. 6, 7, and 8, the fractional optical power lying outside aradius R is plotted for 1550 nm (FIG. 6), 1480 nm (FIG. 7), and 980 nm(FIG. 8), respectively. The refractive index profile is shown by theright ordinate with FIG. 6 for 1550 nm, FIG. 7 for 1480 nm and FIG. 8for 980 nm.

In an optical amplifying or gain fiber, which typically has dopantwithin a portion of or the entire core, the overlap factor Γ, and thus,the gain of various fiber modes with the core region, become different.The gain per unit length of a rare-earth doped optical amplifier isgiven by, g=Γ(N₂σ_(e)−N₁σ_(a)), where Γ is the overlap factor, N_(1,2)are the lower and upper state populations, respectively, and σ_(e,a) arethe emission and absorption cross sections, respectively.

From FIGS. 6, 7 and 8 one can discern the fractional powers (=1−Γ) ofdifferent modes located outside of a 5-micron-radius core withΔN=0.0175. Overlap factor, Γ, which equals the fractional power thatremains inside the core are: 95.9% for LP₀₁, 59.5% for LP₀₂, 88.4% LP₁₁for 74.3% LP₂₁ at 1550 nm. The corresponding values for 980 nm and 1480nm are shown in Table 1.

TABLE 1 WL(μm) LP₀₁ LP₀₂ LP₁₁ LP₂₁ 0.98 98.8% 92.3% 96.7% 93.7% 1.4896.4% 67.4% 89.8% 78.0% 1.55 95.9% 59.5% 88.4% 74.3%

Table 1 shows power content of various modes in a step index core,calculated for different wavelengths.

A fiber amplifier that is based on a nonlinear effect (such as Raman andBrillouin) does not have any rare-earth doping but GeO₂ or otherelements that enhances the Raman or Brillouin gain coefficient. In aRaman amplifier, the gain can be expressed as, g_(R)=G_(R)PΓ/A_(eff).Here, G_(R) is the Raman gain coefficient, P is the pump power, A_(eff)is the effective mode field area of the pump wave, and Γ is thenormalized overlap integral between the pump and signal field.

Due to the dependency of Γ on mode type of the signal wave, the gainexperienced by the different modes can be different in a few-modeamplifier (both the rare-earth and Raman/Brillouin amplifiers).Therefore, there is a need for a few-moded fiber amplifier where themodes will be amplified by a substantially equal amount.

This problem has been solved in accordance with at least one aspect ofthe present invention, by administering mode conversion of each of thelaunched modes into one of the other modes along the fiber amplifier sothat the overall gain experienced by each launched mode becomessubstantially the same. This is further explained and illustrated usingtwo examples shown in FIGS. 9 and 10.

FIGS. 9 and 10 show an amplifier containing six mode converters toconvert three input modes (LP₀₁, LP₁₁, LP₀₂) in the sequences as shownin the figures. FIG. 9 shows a length of fiber after the last converterwhile FIG. 10 shows a length of fiber before the first converter. Thesemode converters are separated by nominally the same length of gainfiber, 1. The mode converters placed within this sequence ensures thateach launched mode assumes the other modes during the course ofpropagation in the amplifier ensuring substantially equal amount ofgain. A mode converter LP_(ab)⇄LP_(cd) thus has a dual function. Itconverts mode LP_(ab) to LP_(cd) and also converts mode LP_(cd) to modeLP_(ab) so that no mode is lost. The overall exponential gain for eachmode becomes 2l(g₀₁+g₁₁+g₀₂) where l also represents the nominal spacingbetween the mode converters and each g_(x,y) represents the gain permeter for the respective mode. Another feature is that when the lengthsare substantially equal, the group delay or dispersion associated witheach mode becomes substantially the same.

When only two modes are involved (e.g., LP₀₁ and LP₁₁), at least twoconverters are needed so that at the exit of the amplifier the modes arerestored. The input modes LP₀₁ and LP₁₁ are transformed into LP₁₁ andLP₀₁, respectively, by the first converter located somewhat at themiddle of the amplifier. The newly-generated LP₁₁ and LP₀₁ are thenrestored back to LP₀₁ and LP₁₁, respectively, by the second converter.

It is possible to convert one mode to the other mode of similarpolarization by applying periodic index perturbations (such as thatshown in FIG. 11). For example, if a mode LP₀₁ and LP₀₂ have effectiverefractive indices of n₀₁ and n₀₂, respectively, a period of therefractive index perturbation is required given byA_(01⇄02)=λ/(n₀₁−n₀₂). FIG. 11 illustrates that a mode converter canalso be made by inscribing long period gratings in the core or in thecladding region in the vicinity of the core.

FIG. 12 shows a schematic diagram of a mode converter interchangingmodes between symmetric LP₀₁ and asymmetric LP₁₁ modes. Specifically,FIG. 12 is a schematic diagram illustrating conversion between LP₀₁ andLP₁₁ modes using microbending. Note that the polarization state ispreserved.

FIG. 13 illustrates the periods of phase perturbations required for modeconversion among four guided modes. Periods are calculated for a fiberwith a step index core, a core diameter of 10 microns, and an indexdifference of 0.0175. One can see that the periodic perturbation thatachieves mode conversion among different modes is different over a widerange of wavelengths, which means that mode conversion would take placebetween only specific pair of modes, thus avoiding cross-talk. FIG. 13thus illustrates periodicity of LPG/microbending that achieves modeconversion between different pairs of modes. The dependency ofperiodicity of the LPGs/microbends as a function of wavelength is shownin further details in the graphs of FIGS. 14 and 15, which illustrateLPG/microbending periodicity for different pairs of modes plotted as afunction of wavelength.

In this simple step-index profile, the six mode converters among thefour guided modes have distinct LPG periods, Λ. As shown in Table 2,below, most of the required LPG periodicities are between 120-350microns. It is possible to tailor the periods of required LPGs byadjusting the refractive index profiles of the multimode fiber. Formulti-wavelength or broadband operation, it is important that gratingperiods remain relatively independent of the signal wavelength. Thisrequires that dΛ/dλ be kept as small a possible, which can be realizedby optimization of the index profile.

For conversion between symmetric modes (e.g., LP₀₁, LP₀₂) and asymmetricmodes (e.g., LP₁₁), a non-symmetric periodic perturbation (e.g., withlateral stress) can be applied to achieve mode conversion. This can bedone efficiently by using periodic microbending with appropriate choicesof bend orientation, for example, as shown in FIG. 12.

TABLE 2 1.55 μm LP₀₁-LP₀₂ LP₀₁-LP₁₁ LP₀₁-LP₂₁ LP₀₂-LP₁₁ LP₀₂-LP₂₁LP₁₁-LP₂₁ Period (μm) 130.8 330.5 147.8 216.6 1139.0 267.4 dΛ/dλ −0.81−107.34 −33.54 43.88 1932.45 −39.50 dΛ/Λdλ −0.0062 −0.3248 −0.22690.2026 1.6966 −0.1477 (1/μm)

Table 2 shows the LPG periodicity and slope of period versus resonancewavelength.

When four modes (generalized as A, B, C and D) exist, the modeconversion can be initiated in the sequences as illustrated in FIG. 16.One can readily appreciate from FIG. 16 that each mode is restored atthe end, and each mode travels one-fourth of the total length. Althoughmode converters are chosen in periodic sequence of A→B→C→D for launchedmode A, other sequences achieved by permutations of the modes (e.g.,A→C→D→B, A→D→B→C, etc.) can be used as well.

The number of mode conversions that each mode has to experience can beexpressed as N(N−1), which increases sharply as N becomes large. Forexample, the number of mode conversions will be 2 for N=2, 6 for N=3,and 12 for N=4.

In the following, another embodiment for mode conversion is shown, whichsignificantly reduces the number of mode transformation that each modeundergoes.

A conversion scheme in accordance with one embodiment of the presentinvention is illustrated in FIG. 17, where each mode converter performsmode conversions between two or more pairs of modes. Consider fourmodes, A, B, C, and D. FIG. 17 shows a cyclic mode conversion shown forN=4. Each set of mode transformers comprises 2 (=N/2) distinctconverters. The total number of conversions that each mode undergoes is4 (=N). Two sets of composite mode converters are formed, Set-I andSet-II performing mode conversion between (A⇄B, C⇄D) and (B⇄C, D⇄A),respectively. When these two sets are placed alternately, a total of Nsuch sets will transform each mode into the other modes and finallyrestore the signal to its original state. Here, the number of modetransformations that each launched mode would undergo is N making thescaling to larger numbers of modes feasible. Note also that in thisscheme one set of modes experiences cyclic mode conversion in onedirection (clockwise: A and C), while the other set experiences suchconversion in the other direction (counter-clockwise: B and D). Thetotal number of discrete mode converters becomes N²/2. It should beappreciated that this scheme is applicable whenever an even number ofmodes is present.

This scheme when applied to six modes is shown in FIG. 18. It is to beunderstood that each mode is transformed only six times in a cyclicfashion. FIG. 18 shows cyclic mode conversion for N=6. Each set of modetransformers comprises three (=N/2) distinct converters. The totalnumber of conversion each mode undergoes for this configuration is six(=N).

In the absence of gain saturation the length of the different gainsections may be the same. However, when the gain is depleted due to pumpdepletion, increasingly longer lengths for successive sections can beselected. As shown in FIG. 19, the length of successive sections of gainfiber is increasingly longer when gain is saturated. It is also possibleto pump each gain section individually by separate pump sources. FIG. 20shows an example of a gain-equalized few-moded fiber amplifier employingmultiple pump sources.

Since the core is multimoded for the signal wave, the pump wave may alsobe split into various modes depending on how it is launched. Therelative intensities of the individual modes can be adjusted so that itis absorbed uniformly inside the core. Few-mode fibers suitable forthese amplifiers may have one or more cores in a single cladding, wherethe core is doped or undoped, and if doped, may be doped with arare-earth dopant, such as Erbium, Ytterbium, and the like, or dopedwith a non rare-earth element, such as, for example, Germanium. One canalso use a rare earth doped fiber with double cladding structures, whichare pumped by a multimode fiber into the inner cladding region and thefew-moded signal will be carried by the core. A double cladding rareearth doped gain fiber allows uniform absorption of pump in the dopedcore. The pump intensity inside the core will be equal to:

(Local_Pump_Power)×(R_(core)/R_(inner) _(—) _(cladding)) ²

The pump and the signal can be combined in a tapered fiber coupler usingon pedestal fibers or beam collimation using bulk or graded index (GRIN)lenses as shown in FIGS. 21 and 22. FIG. 21 illustrates a multimodesignal and pump combiner, while FIG. 22 illustrates a multimode signaland multimode pump combiner for cladding pumping. The few-moded core inthe multiplexed port may be doped with rare earth element(s). Ifundoped, it can be later spliced to a rare earth doped core. The devicesof FIGS. 21 and 22 can be used in the opposite direction to split thesignal and pump. The GRIN lens has a length nominally equal to quarterof the pitch length. The pump wave is launched through the multimodepump port and coupled into the inner cladding of the double claddingfiber at the combined output port.

Although examples are shown for multi-moded amplifiers, it should beclear that this can be used to make a multimode laser, by incorporatingreflectors to provide feedback or connecting in the form of loop. Aschematic diagram of a multi (transverse)-moded fiber laser with alinear cavity is illustrated in FIG. 23, and with a ring cavity in FIG.24.

The methods, fibers and device as provided herein and in accordance withan aspect of the present invention are applied in a multi-channel modedivision multiplexed optical transmission system 2500 as illustrated inFIG. 25. Multiple independent signals, which may be digital opticalsignals, are provided on inputs 2504, 2506, 2508 and 2520 of the system2502, which implements the mode converters and few-moded fibers asdisclosed herein. While four inputs are shown, there may be fewer orthere may be more independent inputs or channels. Each channel ismodulated as a modal state of an optical signal which is transmitted toan output wherein all channels are retrieved and separated according tothe modal states. Each channel is represented as a modal state of anoptical signal that is transmitted to an output. The demodulated signalsare provided on outputs 2505, 2507, 2509 and 2521. In accordance withone embodiment, each channel is represented on an optical output of thesystem 2515 in the same modal state of the optical signal as the modalstate it had at its initial optical input 2514.

Box 2516 represents the fibers and converters as disclosed herein. Inaccordance with one embodiment, all converters and fibers aredimensioned in such a manner that all relevant modal states that werepresent at the input are also present at the output 2515 and that allmodal states have experienced the same or substantially the sametransmission characteristics, which includes gain, loss, group delay,dispersion, etc. In one embodiment, the relative difference betweenmodal states at the output as compared to the relative differences atthe input is preferably less than 10%, more preferably less than 5%.Accordingly, FIG. 25 illustrates a multi-channel mode divisionmultiplexed optical transmission system.

While there have been shown, described and pointed out fundamental novelfeatures of the invention as applied to preferred embodiments thereof,it will be understood that various omissions and substitutions andchanges in the form and details of the methods and systems illustratedand in its operation may be made by those skilled in the art withoutdeparting from the spirit of the invention. It is the intention,therefore, to be limited only as indicated by the scope of the claims.

1. A few-moded optical fiber device to process an input optical signalcontaining N modes, where N is an integer greater than or equal to 2,comprising: an input few-moded fiber enabled to receive the inputsignal; at least N mode converters arranged in a pre-determined order,wherein a first mode converter of the N mode converters is coupled tothe input few-moded fiber, wherein each mode converter transforms onemodal state of the N modal states to a different mode; a few-modedconnecting fiber between each of the N mode converters; and an outputfew-moded fiber coupled to a last mode converter of the N modeconverters to provide an output optical signal containing the N modalstates, wherein each of the N modes in the optical output signal arecharacterized by a substantially identical transmission parameterrelative to corresponding N modes of the input signal.
 2. The device ofclaim 1, wherein the substantially identical transmission parameter isone or more of gain, loss, group delay, and dispersion.
 3. The device ofclaim 1, wherein the optical signal between the input and the outputremains for an equal amount of time in each of the N modes.
 4. Thedevice of claim 1, wherein the connecting fiber is a gain fiber.
 5. Thedevice of claim 1, wherein the connecting fiber is a passive fiber. 6.The device of claim 1, wherein one or more of the N mode converters is along period grating.
 7. The device of claim 1, wherein one or more ofthe N mode converters applies periodic micro bends to generate modeconversion.
 8. The device of claim 1, wherein one or more of the N modeconverters applies a bulk spatial phase modulator.
 9. The device ofclaim 1, wherein one or more of the N mode converters applies a volumephase grating.
 9. The device of claim 1, wherein one or more of the Nmode converters applies a photonic lattice.
 10. The device of claim 1,wherein one or more of the N mode converters applies a periodic indexvariation.
 11. The device of claim 1, wherein the device is applied in amulti-channel optical transmission system.
 12. The device of claim 1,wherein the N mode converters are placed such that at each modeconverter, N/2 pairs of modes are interchanged within respective pairsof modes such that all of the N modes remain distinct from one anotherafter each mode conversion.
 13. The device of claim 1, wherein the Nmodes are restored to the input state after propagating through thetotal of the N mode converters.
 14. The device of claim 1, wherein alength of each of the input, output, and connecting few-moded fibers isthe same.
 15. The device of claim 1, wherein each of the input, output,and connecting few-moded fibers is pumped by a common pump.
 16. Thedevice of claim 1, wherein each of the input, output, and connectingfew-moded fibers is pumped by individual pump waves.
 17. The device ofclaim 1, wherein each of the N modes is a higher order mode.
 18. Thedevice of claim 1, wherein the N modes comprise a fundamental mode andone or more higher order modes.